The present invention relates to an inverter for supplying electrical energy from a DC supply to a load. The invention may also be advantageously used in uninterrupted power supplies and standby power supplies, and is therefore described below also with respect to these applications.
Many forms of inverter circuits are known for converting DC to AC. One known circuit, called the "flyback" type, and described for example in the article "A New Family of Single-Phase and Three-Phase Inverters" by Sayed-Amr El-Hamamsy and R. D. Middlebrook, PCI, October, 1985 Proceedings, Pages 84-98, includes a transformer having a primary winding coupled to the DC supply and a switch for interrupting the DC supply, causing energy to be stored in the transformer, which energy is outputted from the secondary winding.
In the "fly-back" type of inverter, current flows in the secondary winding only when no current flows in the primary winding, and no current flows in thee secondary winding when current flows in the primary winding. The "flyback" type of inverter is therefore to be distinguished from the "forward" (or "push-pull") type inverter, in which current flows in the secondary winding whenever current flows in the primary winding, and no current flows in thee secondary winding when no current flows in the primary winding.
An object of the present invention is to provide an inverter particularly of the "flyback" type, but having a number of advantages, as will be described more particularly below.
Other objects of the invention are to provide an uninterrupted power supply and also a standby power supply utilizing the novel inverter.
According to the present invention, there is provided an inverter for supplying electrical energy from a DC supply to a load, comprising a transformer including a primary winding circuit coupled to the DC supply, and a secondary winding circuit coupled to the load. The primary winding circuit includes a first controlled switch for interrupting the DC supply; and the secondary winding circuit including at least a second, unidirectional controlled switch to produce an output of one sign when closed. The primary winding circuit further includes an electrical device effective to return energy to the DC supply only and always when the first and second switches are open; and a control circuit for separately and independently controlling the operation of the first and second switches to open and close them, at the same frequency, such that during one interval in each cycle the switch in the primary winding is closed to produce an excess of energy which is stored in the transformer, an during another interval in each cycle the switches in the primary winding and the secondary winding are open and the excess energy stored in the transformer is returned to the DC supply. Such an arrangement permits fast and stable control of the inverter.
The invention is particularly useful in, and is therefore described below with respect to, inverters in which thee transformer is a flyback transformer. In such transformers, the primary and secondary winding circuits have polarities such that there is current flow in the secondary winding circuit when no current flows in the primary winding circuit, and no current flow in the secondary winding circuit when there is current flow in the primary winding circuit.
In several preferred embodiments of the invention described below, thee secondary winding circuit includes a third, unidirectional switch poled in the opposite direction as the second switch, such as to produce, when it is the active switch in the secondary winding circuit instead of the second switch, and is closed, an output of the opposite sign as said second switch.
According to another important feature in preferred embodiments of the invention described below, the control circuit includes: means for closing the first switch and opening the second or third switch to start a first Interval in each cycle during which energy is stored in the transformer; means operative at the end of the first Interval to open the first switch and to close the active second or third switch to start a second Interval in each cycle during which energy stored in the transformer is delivered to the load or, in the case of a reactive load, energy in the load is stored in the transformer; and means operative at the end of the second Interval to open all the switches to start a third Interval during which excess energy then stored in the transformer may be returned to the DC supply via the primary winding circuit.
As will be described more particularly below, such an inverter is capable of four-quadrant operation, wherein energy is supplied to the load during the first and third quadrants, and excess energy stored in the transformer or in the load may be returned to the DC supply for recharging it during the second and fourth quadrants. Besides permitting four-quadrant operation, the inverter of the present invention also permits fast and stable control.
The four-quadrant operation of the inverter provides particular advantages when used in an uninterrupted power supply and also in a standby power supply. Thus, it obviates the need for the provision of large separate chargers normally required in such systems in order to keep the back-up power supply fully charged. The invention is therefore described below also with respect to these applications.
Another preferred embodiment of the invention is also disclosed involving a different sequence of control. According to this embodiment, the primary winding control subcircuit opens the first switch at the beginning of each cycle and closes the first switch at a subsequent point in the cycle when, at the end of the respective cycle, the energy stored in the transformer would reach a predetermined value; and the secondary winding control subcircuit closes the second switch at the beginning of the respective cycle and opens the second switch when the voltage at the output of the secondary winding circuit reaches a predetermined value; such that a first interval is started by the opening of the first switch and closing of the second switch, during which energy in the transformer is delivered to the load; a second interval is started by the opening of the second switch during which excess energy in the transformer is returned to the power supply; and a third interval is started by the closing of the first switch during which energy is stored in the transformer.
It will thus be seen that in the first-described embodiment, the interval (therein Interval III) during which excess energy is delivered to the power supply is of fixed termination point; whereas in the latter embodiment this interval (Interval II), is of "floating" duration. That is, Interval II in the latter embodiment starts by the opening of the secondary winding switch (the "second switch"), when the voltage at the output of the secondary winding circuit reaches a predetermined value, and ends by the closing of the primary winding switch (the "first switch") at the point in the cycle when, at the end of the cycle, the energy stored in the transformer would reach a predetermined value. Such a "floating" arrangement for determining the interval during which excess energy stored in the transformer is delivered to the power supply, produces a more efficient and stable operation.
Further features and advantages of the invention will be apparent from the description below.